1. Field of the Invention
This invention relates to apparatus and methods for extracting substantial electrical power from the wind. More particularly, this invention relates to extracting power from winds at relatively high altitudes, e.g. a thousand meters above the surface of the earth.
2. Prior Art
A wide variety of approaches have been devised for generating electricity from the wind. Windmills have been used fairly extensively, but have so far been economically competitive only in special situations such as remote and/or exceptionally windy sites. Present commercial wind energy conversion devices utilize primarily blades mounted on a single shaft, most frequently a horizontal shaft mounted on a tower and pointing into the wind.
U.S. Pat. No. 4,049,300 (Schneider) shows a power-producing apparatus comprising two vertically-displaced parallel axles carrying wheels mounting endless belts to which are secured a series of blades movable around a path having ascending and descending legs; fluid (e.g. air) passing through the apparatus develops power in both legs to provide in effect a two-stage cascaded arrangement. Still another approach is disclosed in U.S. Pat. No. 4,302,684 (Gogins) which shows apparatus comprising a very long closed loop track, with parallel sides and round ends, and carrying cars supporting huge vertical sails. Electricity is generated from movement of the cars around the track caused by the wind.
The above prior art arrangements (which represent only a sampling of the available disclosures) are directed essentially to extracting power from winds at ground level. Thus, their performance suffers from the relatively low power density and extreme variability in time and location of ground-level winds.
There have been proposals made for extracting power from winds at high levels, where the velocity is substantially greater than at ground levels. Such an approach is particularly attractive because the energy content of wind goes up as the cube of the wind velocity. One such system is shown in U.S. Pat. No. 3,924,827 (Lois) which discloses a series of buoyant wings connected by tethers to an electric generator. The system is so arranged that as one wing is being retracted, at least one other wing is being drawn away by the wind to deliver power to the generator.
Another proposal for high-altitude wind power extraction is set forth in a paper entitled "Electricity Generation from Jet Stream Wind" by Fletcher and Roberts, appearing in the July-August, 1979 issue of the Journal of Energy of the AIAA. That scheme comprises an airplane-like structure tethered to an essentially single point on the ground and carrying wind turbines driving electric generators which deliver electric current to the ground through two conductors forming part of the tether. The proposal involves flying such a device at extremely high altitudes, e.g. 12 km. At that height, interference could be expected with piloted aircraft (except possibly in the remote part of Australia considered by the authors of the disclosure).